Valorization of recycled GFRP fibers and flakes from decommissioned boats in concrete: Effects on mechanical and microstructural properties

The widespread use of glass fiber-reinforced polymer (GFRP) across various industries, including boat construction, has resulted in significant waste accumulation. This study expands the application of recycled GFRP in concrete to include flakes, a form not previously explored, alongside fibers. By investigating fibers, flakes, and a hybrid fiber-flake group, this study aims to increase the usable fraction of recycled GFRP in concrete. The effects of recycled GFRP from decommissioned boat hulls on the compressive strength, modulus of elasticity, and splitting tensile strength of concrete were evaluated. Microstructural characteristics were analyzed using mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Incorporating recycled GFRP fibers or hybrid fiber-flake particles enhanced the compressive and splitting tensile strengths of concrete due to fiber bridging, whereas the use of flakes reduced these properties because their geometry and surface characteristics limit effective bonding. At a 2 % volume fraction, fibers increased compressive strength, modulus of elasticity, and splitting tensile strength by 17.8 %, 6.4 %, and 21.6 %, while flakes decreased these properties by 5.5 %, 4.2 %, and 8.4 %, respectively. MIP results showed that flake incorporation led to higher total pore volume and coarser pore structures, consistent with the interfacial defects leading to weaker bonding observed in SEM images, which together explain the reduction in mechanical performance. Although flakes reduced performance, the hybrid group indicated that combining fibers and flakes could accommodate higher recycled GFRP volumes and support more effective and sustainable recycling applications in concrete. • Recycled GFRP fibers or hybrid fiber-flake particles improved concrete strengths. • Recycled GFRP flakes reduced concrete mechanical performance. • SEM revealed weaker flake–cement paste bonds compared to fiber–cement paste bonds. • Flakes with fibers offer a promising path to increase recycled GFRP in concrete.